Historically, it's significant that it went from the least-watched program on tv in its first season to the most-watched show. In its first season, there was one week where it was the literally the least watched original program that was broadcast. It was the number one show regularly later in its run. That's just an unexpected path.

When it started, it aired on NBC, when the joke was that the network was fourth place out of three networks (only ABC and CBS were national networks at the time, FOX didn't exist yet). The only reason it was renewed for a second season was because the brass at NBC liked the show and thought its quality was better than anything in development. During its summer reruns (remember, only three broadcast networks at the time, so most of what it was airing against would have been watched already), audiences discovered it and stuck around for its second season. It's an example pointed to when networks want to cancel quality shows that haven't yet found an audience.

With Sam and Diane, it introduced the concept of a season(s)-length plot to a genre where the standard was to keep every episode self-contained. The conventional wisdom was that audiences would be lost if they missed an episode, and other than summer re-runs there was no way to rewatch an episode, so you would lose audiences if character relationships changed or plots carried over more than a very rare two-parter. It turns out audiences were smarter than they were given credit for and also they became more engaged with the long-running will-they-won't-they plot.

You can draw a direct line from Sam/Diane to Ross/Rachel, Niles/Daphne, Jim/Pam, Leonard/Penny etc. as a concept introduced in Cheers has become a de facto genre element for network sitcoms.

As Cheers grew in popularity, it became the anchor of NBC's broadcast schedule. There is no Must-See-TV block in the 1990s without Cheers in the 1980s.

Random side note: its creators wanted to name the production company A Jew and Two Mormons but ended up switching it to Charles/Burrows/Charles. The Charles brothers were raised Mormon.